The present disclosure relates to fill level devices that employ ultrasonic and/or electromagnetic measuring signals, such as radar based measuring devices. Such fill level measuring devices are offered by the Endress+Hauser group of companies under the trademarks Prosonic, Micropilot and Levelflex, for example. A fill level measuring device in the sense of the present disclosure is not to be understood as being restricted to a unitary collection of hardware components but can also be a system of spatially separated units. A measurement device can be viewed as having a transducer and a transmitter, wherein the transducer serves to convert a process variable, such as the fill level of a material in a tank, into an electrical signal, and wherein the transmitter serves to sample and process this electrical signal in order to produce a value for the process variable that corresponds to a physical situation that is to be measured. The transmitter, as the case may be, can further serve to transmit and/or save the determined process value for further use. The transducer generally includes a microprocessor and/or microcontroller and various other electrical and electronic circuits. The transducer and the transmitter can be concentrated in a single unitary device, or they can be spatially separated. In the case where they are spatially separated, some sort of communication means, such as a cable or a wireless communication means, is provided. The distinction between transmitter and transducer cannot always be strictly applied, as the transducer in some cases serves to preprocess a measurement signal and sometimes even includes a microprocessor.
Ultrasonic and radar based measuring devices come in a wide variety in terms of measurement accuracy, reliability and cost. These aspects of a fill level measuring device are at least in part determined by the quality and/or characteristics of the components used in the construction of the device. In today's market, fill level measuring devices generally employ narrow-bandwidth band-pass filters to filter out noise components outside of the frequency range of the intermediate frequency signal as well as analog to digital converters for sampling the intermediate frequency signal that have a high sampling rate. A high sampling rate in the sense of the present disclosure is a sampling rate that is at least twice the frequency of the intermediate frequency signal, but can also be much higher. The intermediate frequency signal in the case of an ultrasonic measuring device is to be understood as being the frequency of a signal output by an ultrasonic converter in the ultrasonic measuring device. An application of narrow-bandwidth band-pass filters for an ultrasonic fill level device is described in the German Patent Publication DE00102009026885A1, for example, wherein the trade-off between bandwidth and noise in the selection and application of band-pass filters is described. The analog intermediate frequency signal of an ultrasonic or radar based fill level measuring device generally includes a signal frequency between 0 kHz and 200 kHz, in particular between 10 kHz and 100 kHz.
High speed analog to digital converters and narrow-band band-pass filters, as well as components such as logarithmic amplifiers are generally expensive and have high energy needs. In the German Patent Publication DE102006006572A1, it is suggested that sampling and digitalization of an intermediate frequency signal is possible, in principle, wherein the requirements of the Nyquist-Shannon sampling theorem are not fulfilled in order to save energy as well as to reduce the memory and computational requirements of the a fill level measurement device. This can be implemented so that the difference in time between sampled digital values, said values being alternatingly assigned to one of two groups, is larger than the recipricol of twice the intermediate frequency. The requirement of the Nyquist-Shannon sampling theorem that is no longer met due to this digital sorting of sampled values requires that the sampling rate be at least twice the frequency of the signal that is to be sampled.